Flexible coating resins from siloxane resins having a very low degree of organic substitution

ABSTRACT

Abrasion resistant siloxane resins having a low degree of organic substitution thereby tending to be brittle can be made flexible by including some degree of phenyl substitution into the organic substitution.

This is a division of application Ser. No. 863,970, filed Dec. 23, 1977,now abandoned. U.S. application Ser. No. 903,467, filed May 8, 1978 is acopending application.

BACKGROUND OF THE INVENTION

The availability of lightweight plastics has led to the replacement ofglass by such plastics for numerous uses. Over the past several years,new plastics have been developed which find use in window glazing,lenses, clear face shields, aircraft canopies and the like. Althoughsuch plastics have many outstanding properties, they are deficient intheir resistance to scratching. An outstanding example is thedeterioration of plastic sunglass lenses by common everyday use becausesuch glasses are frequently removed from the face and laid on hardsubstrates, lens down.

Thus, in order to better utilize the advantageous properties of today'splastics, there is a need to render such plastics scratch and abrasionresistant.

In order to obtain abrasion resistant surfaces, such as, for example,polycarbonate surfaces, investigators have tended to coat very thincoats of organic or silicone resins on the surface of the plastics. Theintent was to obtain abrasion resistance without losing the opticalproperties of the plastic substrate.

Such an organic coating is disclosed in U.S. Pat. No. 4,018,941. Such anorganic coating is prepared from polyols and urethanes and is cured viamelamine crosslinkers. Although some degree of abrasion resistance isafforded by the melamine coating, it has a tendency to be affected byoutdoor exposure and eventually the coating deteriorates.

In view of the above, silicone coatings which have a tendency to be veryweather resistant were developed. Such resins are shown in U.S. Pat.Nos. 3,389,114, 3,389,121, 3,634,321, 3,642,698 and 3,935,346, allassigned to Owens-Illinois. The latter patent teaches a method of makingan abrasion resistant coating from an alkylated melamine-formaldehyderesin and a hydrolyrate of MeSi(OR)₃. These resins all have good weatherresistance but rather moderate abrasion resistance.

There was developed later on a siloxane resin having a low degree oforganic substitution. The coating had hardness and therefore goodabrasion resistance. Such resins are disclosed in U.S. Pat. No.3,986,997 issued Oct. 19, 1976 to Harold A. Clark and assigned to theDow Corning Corporation. The Clark resins are very versatile materialsand find utility as abrasion resistant coatings on a number ofsubstrates which require good abrasion resistance.

The only disadvantage of the Clark resins is the fact that they tend tobe inflexible, that is, under certain circumstances the coatings tend tocraze.

It is well known in the silicone art that flexibility can be built intoa siloxane resin coating by merely incorporating a dimethyl containinghydrolyzable silane in the formulation when the resin is first prepared(see Canadian Pat. No. 1,015,888). Unfortunately, the presence ofdimethyl siloxane in any siloxane resin tends to also soften the coatingso that the abrasion resistance falls off. Thus, for purposes ofobtaining an abrasion resistant coating with flexibility, one would notsuggest using the above approach in preparing the resins.

What is needed is a weather resistant, abrasion resistant, flexible,clear coating.

THE INVENTION

There has now been discovered a means of improving the flexibility ofsiloxane resins having a low degree of organic substitution withoutsacrificing a great deal of the abrasion resistance of the coating.

What is disclosed herein is an improvement in the flexibility of theClark resins shown in the U.S. Pat. No. 3,986,997 set out above.

Such improved flexibility can be obtained by incorporating in theoriginal formulation for the Clark resin a certain amount ofmonophenylsilsesquioxane structure.

It has been found that the incorporation of φSi(OH)₃ in the Clark resingives increased flexibility to the resin without significant loss ofabrasion resistance of the cured coating.

Not only can one obtain flexibility in siloxane resins having a lowdegree of substitution, one can obtain control over the degree offlexibility given to such resins by merely controlling the amount ofmonophenyl put into the formulation i.e. the degree of flexibility builtinto the cured coating of the resin is linearly dependent on the amountof monophenyl actually incorporated in the formulation. The control issuch that flexibility±5% can be estimated from the amount of monophenylincorporation.

At least 1 weight percent of φSi(OH)₃, based on the weight of totalRSi(OH)₃ present in the composition, is required to get the flexibilityeffect. Up to 30 weight percent of φSi(OH)₃ can be utilized. Generally,greater than 30 weight percent, even though giving increasedflexibility, does not retain the required abrasion resistance.

The resins are prepared by the methods found in the above Clark patentand the only difference is that φSi(OMe)₃, in the proper proportions, ismixed with the CH₃ Si(OH)₃ before the hydrolysis and contact with thecolloidal silica. The φSi(OMe)₃ can be pre-hydrolyzed before mixing withthe CH₃ Si(OH)₃ but no significant advantage is obtained thereby.

The silica component of the composition is present as colloidal silica.Aqueous colloidal silica dispersions generally have a particle size inthe range of 5 to 150 millimicrons in diameter. These silica dispersionsare prepared by methods well-known in the art and are commerciallyavailable under such registered trademarks as "Ludox" and "Nalcoag". Itis preferred to use colloidal silica of 10-30 millimicron particle sizein order to obtain dispersions having a greater stability and to providecoatings having superior optical properties. Colloidal silicas of thistype are relatively free of Na₂ O and other alkali metal oxides,generally containing less than 2 weight percent, preferably less than 1weight percent Na₂ O. They are available as both acidic and basichydrosols. Colloidal silica is distinguished from other waterdispersable forms of SiO₂, such as nonparticulate polysilicic acid oralkali metal silicate solutions, which are not operative in the practiceof the present invention.

The silica is dispersed in a solution of the siloxanol carried in alower aliphatic alcohol-water, or ether ester-water, cosolvent. Suitablelower aliphatic alcohols include methanol, ethanol, isopropanol, andt-butyl alcohol. Mixtures of such alcohols can be used. Isopropanol isthe preferred alcohol and when mixtures of alcohol are utilized it ispreferred to utilize at least 50 weight percent of isopropanol in themixture to obtain optimum adhesion of the coating. Suitable ether estersare ether esters of ethylene or propylene glycol such as CH₃ COO(CH₂ CH₂O)₂ C₂ H₅, CH₃ COO(CH₂ CH₂ O)₂ C₄ H₉, CH₃ COOCH₂ CH₂ OC₂ H₅, CH₃ COOCH₂CH₂ OCH₃ and CH₃ COOCH₂ CH₂ OC₄ H₉ and analogs of such materialsprepared from propylene glycol. The solvent system should contain fromabout 20 to 75 weight percent of alcohol or ether ester to ensuresolubility of the siloxanol. Optionally one can utilize an additionalwater-miscible polar solvent, such as acetone, butyl cellosolve and thelike in a minor amount, for example, no more than 20 weight percent ofthe cosolvent system.

To obtain optimum properties in the coating and to prevent immediategellation of the coating composition, sufficient acid to provide a pH offrom 2.8 to 5.2 must be present. Suitable acids include both organic andinorganic acids such as hydrochloric, acetic, chloroacetic, citric,benzoic, dimethylmalonic, formic, glutaric, glycolic, maleic, malonic,toluene-sulfonic, oxalic and the like. The specific acid utilized has adirect effect on the rate of silanol condensation which in turndetermines shelf life of the composition. The stronger acids, such ashydrochloric and toluenesulfonic acid, give appreciably shortened shelfor bath life and require less ageing to obtain the described solublepartial condensate. It is preferred to add sufficient water-misciblecarboxylic acid selected from the group consisting of acetic, formic,propionic and maleic acids to provide pH in the range of 4 to 5.5 in thecoating composition. In addition to providing good bath life, the alkalimetal salts of these acids are soluble, thus allowing the use of theseacids with silicas containing a substantial (greater than 0.2% Na₂ O)amount of alkali metal or metal oxide.

The composition is easily prepared by adding the trialkoxysilanes, suchas R'Si(OCH₃)₃, to colloidal silica hydrosols and adjusting the pH tothe desired level by addition of the organic acid. The acid can be addedto either the silanes or the hydrosol prior to mixing the two componentsprovided that the mixing is done rapidly. The amount of acid necessaryto obtain the desired pH will depend on the alkali metal content of thesilica but is usually less than one weight percent of the composition.Alcohol is generated by hydrolysis of the alkoxy substituents of thesilane, for example, hydrolysis of one mole of --Si(OC₂ H₅)₃ generates 3moles of ethanol. Depending upon the percent solids desired in the finalcomposition, additional alcohol ether ester, water or a water-misciblesolvent can be added. The composition should be well mixed and allowedto age for a short period of time to ensure formation of the partialcondensate. The coating composition thus obtained is a clear or slightlyhazy low viscosity fluid which is stable for several days.

Buffered latent condensation catalysts can be added to the compositionso that milder curing conditions can be utilized to obtain the optimumabrasion resistance in the final coating. Alkali metal salts ofcarboxylic acids, such as potassium formate, are one class of suchlatent catalysts. The amine carboxylates and quaternary ammoniumcarboxylates are another such class of latent catalysts. Of course thecatalysts must be soluble or at least miscible in the cosolvent system.The catalysts are latent to the extent that at room temperature they donot appreciably shorten the bath life of the composition, but uponheating the catalyst dissociates and generates a catalytic speciesactive to promote condensation. Buffered catalysts are used to avoideffects on the pH of the composition. Certain of the commerciallyavailable colloidal silica dispersions contain free alkali metal basewhich reacts with the organic acid during the adjustment of pH togenerate the carboxylate catalysts in situ. This is particularly truewhen starting with a hydrosol having a pH of 8 or 9. The compositionscan be catalyzed by addition of carboxylates such as dimethylamineacetate, ethanolamine acetate, dimethylaniline formate,tetraethylammonium benzoate, sodium acetate, sodium propionate, sodiumformate or benzyltrimethylammonium acetate. The amount of catalyst canbe varied depending upon the desired curing condition, but at about 1.5weight percent catalyst in the composition, the bath life is shortenedand optical properties of the coating may be impaired. It is preferredto utilize from about 0.05 to 1 weight percent of the catalyst.

To provide the greatest stability in the dispersion form while obtainedoptimum properties in the cured coating, it is preferred to utilize acoating composition having a pH in the range of 4-5 which contains 10-35weight percent solids; the silica portion having a particle size in therange of 5-30 millimicrons, the partial condensate CH₃ Si(OH)₃ andφSi(OH)₃ being present in an amount in the range of 35 to 55 weightpercent of the total solids in a cosolvent of methanol, isopropanol andwater or CH₃ COOCH₂ CH₂ OCH₃ and water or ether esters, the alcoholsrepresenting from 30 to 60 weight percent of the cosolvent and acatalyst selected from the group consisting of sodium acetate andbenzyltrimethylammonium acetate being present in an amount in the rangeof 0.05 to 0.5 weight percent of the composition. Such a composition isrelatively stable and, when coated onto a substrate, can be cured in arelatively short time at temperatures in the range of 75°-125° C. toprovide a transparent abrasion resistant surface coating.

The coating compositions of the invention can be applied to solidsubstrates by conventional methods, such as flowing, spraying or dippingto form a continuous surface film. Although substrates of soft plasticsheet material show the greatest improvement upon application of thecoating, the composition can be applied to other substrates, such aswood, metal printed surfaces, leather, glass, ceramics and textiles. Thecompositions are especially useful as coatings for dimensionally stablesynthetic organic polymeric substrates in sheet or film form, such asacrylic polymers, for example, poly(methylmethacrylate), polyesters, forexample poly(ethyleneterephthalate) and polycarbonates, such aspoly(diphenylolpropane)carbonate, polyamides, polyimides, copolymers ofacrylonitrile-styrene, styrene-acrylonitrile-butadiene copolymers,polyvinyl chloride, butyrates, polyethylene and the like. Transparentpolymeric materials coated with these compositions are useful as flat orcurved enclosures, such as windows, skylights and windshields,especially for transportation equipment. Plastic lenses, such as acrylicor polycarbonate ophthalmic lenses, can be coated with the compositionsof the invention. In certain applications requiring high opticalresolution, it may be desirable to filter the coating composition priorto applying it to the substrate. In other applications, such ascorrosion-resistant coatings on metals, the slight haziness (less than5%) obtained by the use of certain formulations, such as thosecontaining citric acid and sodium citrate, it not detrimental andfiltration is not necessary.

By choice of proper formulation, including solvent, applicationconditions and pretreatment of the substrate, the coatings can beadhered to substantially all solid surfaces. A hard solvent-resistantsurface coating is obtained by removal of the solvent and volatilematerials. The composition will air dry to a tack-free condition, butheating in the range of 50° to 150° C. is necessary to obtaincondensation of residual silanols in the partial condensate. This finalcure results in the formation of silsesquioxanes of the formulaφSiO_(3/2) and RSiO_(3/2) and greatly enhances the abrasion resistanceof the coating. The coating thickness can be varied by means of theparticular application technique, but coatings of about 0.5 to 20 micronpreferably 2-10 micron thickness are generally utilized. Especially thincoatings can be obtained by spin coating.

Now so that those skilled in the art can better understand andappreciate the invention, the following examples are presented.

In the examples and elsewhere in this disclosure, the use of the symbolsφ and Me mean "phenyl" and "methyl" respectively.

EXAMPLE 1 Preparation of phenyl containing resins of this invention.

Six resins were prepared for evaluation.

Sample 1 was prepared according to the procedure of Example 1 of U.S.Pat. No. 3,986,997 and was used for comparison purposes.

Samples 2-6 were prepared according to the following procedure. Samples2, 3 and 4 fall within the scope of this invention and Samples 5 and 6fall outside the scope of the claims.

    5% φSi(OH).sub.3 /45 CH.sub.3 Si(OH).sub.3

To a three-necked, round-bottomed flask was added 154.5 grams of acolloidal silica having an initial pH of 3.1 containing 34% SiO₂ ofapproximately 22 millimicron particle size and having an Na₂ O contentof less than 0.01 weight percent. It was cooled to 8° C. and 5.3 gramsof glacial acetic acid was added. 96.0 grams of CH₃ Si(OMe)₃ and 8.1grams of φSi(OMe)₃ was premixed and slowly added to the colloidal silicawith vigorous stirring and external cooling. The methoxy silanes wereallowed to hydrolyze with the formation of methanol. After thehydrolysis was complete, 2.7 grams of a 10% solution of sodium acetateand 132.4 grams of isopropanol were added. After seven days standing,66.2 grams additional alcohol was added and the solution filtered.

EXAMPLE 2

Plexiglass® panels 4"×4"×1/8" (10.16 cm×10.16 cm×0.32 cm) after beingcleaned with isopropanol and air dried, were flow coated with a 22.5%solids resin, allowed to air dry and then cured for 18 hours at 75° C.

Similar 1"×4"×1/8" (2.54 cm×10.16 cm×0.32 cm) strips were prepared totest the flexibility. The test for flexibility is relative and wascarried out in the following manner.

The 1" (2.54 cm) wide strips were placed in a vise-like device so thatthe longest i.e. 4" (10.16 cm) axis was horizontal. A strong light wasplaced on the opposite side of the strip from where the observation wasbeing made, so that the craze marks when they formed, were more easilyseen. The vise-like device is manipulated by hand to draw the vise jawstogether slowly so that the plastic strip first humps in the center andthen begins to form a semi-circle with the coating on the outside of thesemi-circle. While observing the coating, the jaws are moved slowlytogether (decreasing the radius of curvature) until craze marks arepropogated in the coating. When the craze marks show across the entirewidth of the plastic strip, the end point has been reached.

The degree of flexibility is then calculated in the following manner.The initial length of the strip before compression is designated AB. Thedistance between the vise jaws at the end of compression is designatedAB. Prior measurements of angle of θ plotted versus AB/AB give a graphi.e. θ=AB/AB from which θ can be easily determined.

The radius of curvature (r) can then be calculated. ##EQU1##

One has to assume that AB is an arc of a circle. The actual shape of thesemi-circle in this test is a parabola which suggests that this is amore severe test than a test which had a true semi-circular shape.

The abrasion resistance was determined according to ASTM MethodD1044-56. The instrument was the Tabor A braser. A 500 gram test loadwas used with CS-10F abrasive wheels and the test panels subjected to500 revolutions on the abraser turntable. The percent change in hazewhich is the criterion for determining the abrasion resistance of thecoating is determined by measuring the difference in haze of theunabrased and abrased coatings. Haze is defined as that percentage oftransmitted light which in passing through the specimen deviates fromthe incident beam by forward scattering. In this method, only light fluxthat deviates more than 2.5 degrees on the average is considered to behaze. The .increment. Haze on the coatings was determined by ASTM MethodD1003-61. A Hunter Haze Meter: Gardner Laboratory, Inc. was used. The.increment. Haze was calculated by measuring the amount of diffusedlight dividing by the amount of transmitted light and multiplying by onehundred.

                  The Results                                                     ______________________________________                                                        Partial                                                                       Conden-                                                                       sate     Adhe- Average of                                                     Formu-   sions to                                                                            Two results                                    Sample                                                                              φ wt. %                                                                             lation   Plexi-         Abrasion                              No.   total solids                                                                            CH.sub.3                                                                             φ                                                                             glas  radius in cm.                                                                          Δ  Haze                       ______________________________________                                        1      0        100     0  100%  10.16    2.5%                                2      5        90     10  100%  7.87     3.7%                                3     10        80     20  100%  8.64     4.6%                                4     15        70     30  100%  8.13     5.0%                                5     20        60     40  100%  7.62     7.9%                                6     25        50     50  100%  6.60     9.2%                                ______________________________________                                    

The adhesion test was the 1/8" crosshatch tape pull test in which thecured coating is crosshatched in 1/8" squares using a sharp object, overa square inch area. Adhesive tape (#600 Adhesive-3M Company) is firmlypressed onto the crosshatched area and sharply pulled away. If all ofthe coating remains, the adhesion is 100%.

That which is claimed is:
 1. An article comprising a solid substratecoated with a pigment-free aqueous coating composition comprising adispersion of colloidal silica in a lower aliphatic alcohol-watersolution, or in an ether ester of ethylene or propylene glycol-watersolution, of the partial condensate of a mixture of silanols of theformula RSi(OH)₃ in which R is selected from the group consisting ofalkyl radicals of 1 to 3 inclusive carbon atoms and phenyl, at least 70weight percent of the silanol being CH₃ Si(OH)₃, at least 1 weightpercent of the silanol being of the formula phenylSi(OH)₃, saidcomposition containing 10 to 50 weight percent solids consistingessentially of 10 to 70 weight percent colloidal silica and 30 to 90weight percent of the partial condensate, said composition containingsufficient acid to provide a pH in the range of 2.8 to 6.0.
 2. Anarticle in accordance with claim 1 where in the coating composition, theacid is a water-miscible organic acid selected from the group consistingof acetic, formic, propanoic and maleic acid.
 3. An article inaccordance with claim 2 wherein the coating composition contains fromabout 0.05 to 1.5 weight percent of a buffered latent silanolcondensation catalyst.
 4. An article in accordance with claim 3 whereinsaid solid substrate is transparent.
 5. An article in accordance withclaim 4 wherein said solid substrate is acrylic polymer.
 6. An articlein accordance with claim 4 wherein said solid substrate is polyester. 7.An article in accordance with claim 6 wherein the polyester ispoly(diphenylol propane) carbonate.
 8. An article in accordance withclaim 6 wherein the polyester is poly(diethylene glycol bis allyl)carbonate.
 9. An article in accordance with claim 4 in the form of alens.
 10. An article in accordance with claim 8 wherein the lens is anophthalmic lens.
 11. An article in accordance with claim 1 wherein thesubstrate is a metal substrate.
 12. An article as claimed in claim 11wherein the metal substrate is selected from a group consisting ofsilver, copper, aluminum, titanium and stainless steel.
 13. An articleas claimed in claim 1 wherein the coating is cured.